Angular Leaf Spot Disease on Strawberry and Its Control

By Jim Mertely,GCREC- Balm and Craig Chandler, GCREC- Balm

Xanthomonas fragariae, the bacterium that causes angular leaf spot (Fig. 1), is present in many commercial strawberry fields in Florida .  X. fragariae multiplies prolifically in the small angular spots it produces on diseased leaves.  The bacterium oozes out of the spots on the underside of the leaf as a viscous secretion.  Development of the pathogen is favored by mild humid days (65-70º F) and cold nights with near-freezing temperatures, while spread is facilitated by rainfall, overhead irrigation, and harvest operations.  Serious losses may occur during epidemics when fruit caps (calyces) are spotted or dry up in systemically infected plants.    
         Angular leaf spot is ideally controlled by the use of healthy transplants to exclude the bacterium from newly-planted fields.  Most epidemics are thought to originate from infected planting materials because X. fragariae is restricted to strawberry, and does not persist well in annual strawberry if crop residues are disposed of properly.  Control measures are limited once the disease appears in the field.  Our current control recommendations are listed below:

1. Avoid harvesting and moving equipment through the field when the plants are wet.
2. Harvest fields showing the least amount of symptoms first; move progressively to the field showing the most symptoms.
3. Monitor weather conditions closely, and use sprinkler irrigation only when freeze protection is absolutely necessary.
4. Use copper-based bactericides judiciously.  While these products should suppress the disease, they may also reduce yields if overused. 
5. When spraying products for angular leaf spot control, good coverage is necessary since most inoculum is produced and spread from the underside of the leaf. However, the use of spreader-stickers should be minimized since adjuvants of this type facilitate entry of the bacterium into the leaf and may increase disease severity.  

More details about angular leaf spot can be found at http://strawberry.ifas.ufl.edu , under “Plant Pathology Fact Sheets” and “Digital Library of Diseases”. 

Fig. 1. A strawberry leaf showing symptoms of angular leaf spot disease.